The invention relates generally to vehicle transmission control devices and more specifically to a combined control device for a main transmission and auxiliary, two-speed range box.
Manual transmissions having two or three shift rails which provide reverse as well as three or five forward gear ratios, respectively, provide appropriate and sufficient gear ratio ranges and forward speeds to fulfill the requirements of a vast majority of transmission applications. Transmissions providing greater numbers of gear ratios, however, are routinely produced but their size, complexity and cost is representative of their specialization and rather more limited application.
Devices do exist, however, which extend the gear ratio range and thus capabilities of, for example, transmissions having five forward gears. Such devices are auxiliary gear or range boxes which are operably disposed in tandem with the main transmission. Typically, such auxiliary gear boxes provide two gear ranges generally designated high and low which, when combined with the several ranges of the main transmission, effectively double the number of gear ratios and forward speeds which the compound transmission assembly provides. In practice, several of the plural ranges provided by the transmission when the auxiliary gear box is in the lower range typically overlap those provided when the auxiliary box is in its higher range. Often times then, the auxiliary gear box is utilized to augment the higher speed gears of the main transmission by providing still lower overall gear ratios through the transmission assembly and thus improve fuel economy and highway performance. Appropriate selection and combination of lower main transmission gear ratios and range box ratio can, however, readily provide non-overlapping overall gear ratios when the low to high range box shift accompanies a shift between lower gears.
Operator selection of the gear ratio provided by the auxiliary gear box may be accomplished by positioning a two position switch which in turn controls a mechanical transducer in the auxiliary box to select the desired range. The switch may be electric, pneumatic or hydraulic and controls a transducer of the corresponding type disposed at the auxiliary gear box. Frequently, such switch is secured directly to the shift lever such that the operator may select both the gear of the main transmission and the range of the auxiliary transmission with one hand. Such a configuration, however, permits independent selection of the gear ratios of both the main transmission and auxiliary gear box thus generally permitting the gear ratio overlap discussed above. While this situation is not specifically undesirable, it does tend to promote unnecessary gear shifts, clutch wear and less than optimum fuel economy and performance. This is to say nothing of the inconvenience created by the often complex motion required to simultaneously move both the shift lever and the range selector switch attached thereto. Another difficulty of such independent control over the main and auxiliary gear boxes relates to the possibility of inadvertently effecting a change in the ratio of the auxiliary gear box while the clutch is engaged and power is being transmitted through the transmission assembly. Since such a shift is undesirable, it is considered to be beneficial to include means whereby a change between the ratios of the auxiliary gear box is prohibited unless the main transmission is in neutral and thus not transferring power.
Various linkages and shift patterns have been proposed in mechanisms which combine the operation of and control over both the main and auxiliary gear boxes with a single control having a complex shift pattern. For example, U.S. Pat. No. 3,975,970 to Elfes et al. discloses a mechanism whereby a single operating lever both selects one of four gears provided by the main transmission and one of the two ranges provided by an auxiliary gear box. Forward and reverse is independently selected by an additional shift control and independent mechanism. U.S. Pat. No. 2,694,943 to Brumbaugh discloses a complex shift control linkage having pivotally disposed, as well as axially sliding, linkage components which transfer conventional shift lever motion through both vertical and horizontal distance as well as a right angle.